Planetary wave breaking and nonlinear reflection: Seasonal cycle and interannual variability

Forty-six years of daily averaged NCEP-NCAR reanalysis data are used to identify the occurrence of planetary wave breaking (PWB) in the subtropical upper troposphere. As large-amplitude waves propagate into the subtropics where the zonal flow is weak, they may break. PWB is diagnosed by observing the large-scale meridional overturning of potential vorticity (PV) contours on isentropic surfaces near the subtropical tropopause. PWB occurs most often during summer, and almost exclusively over the subtropical ocean basins in the Northern Hemisphere. The seasonal evolution of the zonal flow (and the associated latitudinal PV gradient) regulates the location and frequency of PWB. Significant interannual variability in PWB is associated with well-known modes of climate variability. One of the most interesting dynamical consequences of PWB is the possibility of nonlinear reflection poleward out of the wave-breaking region. Modeling studies have found nonlinear reflection following PWB. Observations show that about 36% of all PWB events are followed by nonlinear reflection back into midlatitudes. In these cases, a poleward-arching wave train can be seen propagating away from the wave-breaking region following breaking. It is suggested that a sufficiently strong latitudinal PV gradient must be present downstream of the wave-breaking region for reflection to take place. The proportion of PWB events that is reflective stays rather constant through the year, with slightly higher numbers in spring and fall compared to those in winter and summer.